1. Field of the Invention
The present invention relates to a method of controlling the speed of a piston in an injection molding apparatus such as a die casting machine or an injection molding machine.
2. Description of the Prior Art
A conventional injection molding apparatus with a mechanism for controlling the speed of the piston in an injection cylinder includes an injection cylinder and an injection plunger, for casting a molten metal into a mold, connected to a piston rod of the injection cylinder through a coupling. A striker is connected integrally to the piston rod or coupling to turn on and off a limit switch. A signal of the limit switch is put in a limit switch signal detector. A system for detecting the position of the plunger is constructed by the above-mentioned striker, limit switch, and limit switch signal detector. A signal from the limit switch signal detector is put into a control signal generating system. When the injection plunger arrives at a predetermined position, the flow rate adjusting valve is opened or closed to a degree corresponding to a predetermined value set in advance in the adjusting signal generating system by a speed setting device.
It has hithertofore been known that there is a certain time delay between the point of actuation of the limit switch and the point of the start of the operation of a movable part of the flow rate adjusting valve, such as a spool. Namely, it has been considered that when a relay in a control panel is actuated in succession, electric and mechanical delays or time deviations due to delays of electric signals and operation delays of the flow rate adjusting valve are caused. In fact, when a solenoid is actuated, it takes a certain time exceeding a predetermined time for the movable part, such as the spool, to start the operation. There are deviations of this delay time and there are also deviations of the stroke of the movable part. In a conventional apparatus, in fact, when the limit switch is actuated, there is a delay time of about 20 to about 100 msec before the speed of the piston begins to change. It has conventionally been considered that this time delay is due only to electric and mechanical delays.
Japanese Utility Model Application No. 57-22, to which U.S. patent application Ser. No. 455.512 corresponds, discloses a special pulse-motor-driven flow rate adjusting valve designed to minimize electric and mechanical delays. This delay is included in an operation delay described later. According to this technique, the electric and mechanical delays for the start of opening of a spool of this special flow rate adjusting valve after the actuation of the limit switch can be controlled through a microprocessor to less than 1 msec.
When experiments were carried out using this apparatus, however, it was found that when this flow rate adjusting valve was actuated, there was still a time delay of about 10 to about 50 msec before the piston speed began to change. It was found that this time delay was caused by the inertia of the mechanical portion and operation oil, which requires time for acceleration, and also the viscousness and compressibility of the operating oil. This time delay cannot be eliminated. Accordingly, when the injection speed is changed, instructions for controlling the speed should be taking such a time delay into consideration. The operation delay consists of such time delay and the above-mentioned electric and mechanical delays.
This type of control requires continuous adjustment of the timing of instructions for starting operation of the flow rate control valve. According to the conventional technique, this adjustment was mainly determined by perception of individual workers as complicated measurement operations would otherwise be necessary. While this has been possible in the past, recently, shapes of molded articles have become more complicated, and improvement and standardization of quality have become necessary. Accordingly, delicate and highly precise adjustment is needed.